Congenital heart disease (CHD) is the most common type of birth defect worldwide. Mutations in PTPN11, the gene encoding the protein tyrosine phosphatase (PTP) Shp2, are directly implicated in CHD, causing ~50% of Noonan Syndrome (NS) and nearly all LEOPARD Syndrome (LS) cases. Both autosomal dominant, NS and LS are allelic variant disorders that have several phenotypic characteristics in common, including cardiac defects. Despite these similarities, the biochemical properties of Shp2 phosphatase function between NS and LS are very different; whereas PTPN11 NS mutations are gain-of-function activating mutations, LS mutations are loss-of-function and behave as dominant negatives. Precisely how opposing PTP catalytic functions lead to such similar disease etiologies remains unknown. We propose that differences in NS and LS are mediated by phosphatase-dependent functions, whereas similarities are mediated by phosphatase- independent functions in PTPN11. We hypothesize that LS mutations evoke both phosphatase- dependent and -independent functions of Shp2 to induce aberrant signaling effects during cardiac development that lead to the onset of hypertrophic cardiomyopathy (HCM). To address this directly, we generated inducible knockin mice expressing the LS-associated Ptpn11 Y279C mutation. When crossed to deleter-Cre, these Shp2LS/+ mice recapitulate nearly all aspects of the human LS phenotype, including progressive HCM. Importantly, Shp2LS/+ mice show LS mutations are catalytically inactive in vivo. Therefore, as expected, there are aberrantly regulated phosphatase-dependent mechanisms associated with LS. Heart lysates from Shp2LS/+ mice have abrogated agonist-evoked Erk/Mapk activity, in contrast to Shp2NS/+ mice, whose heart lysates show elevated Erk/Mapk signaling. Moreover, Shp2LS/+, but not Shp2NS/+, heart lysates have elevated basal and agonist-induced Akt and mTor activity. However, these differences do not account for the similarities in LS and NS phenotypes. We propose that phosphatase-independent signaling must also exist and contribute to the cardiac defects in NS and LS. Indeed, with our established mouse model systems on hand (LS, Shp2 heterozygous null, Shp2 floxed, and NS mice), we have the unique opportunity to parse phosphatase-dependent vs. -independent roles for Shp2 in LS by investigating the signaling similarities/differences in LS and NS during cardiac development. We propose to 1) determine (and compare to NS) the in-vivo effects of LS expression in lineage-specific cell types during cardiac development and 2) parse the phosphatase-dependent vs. -independent contributions of Shp2 in the heart. We expect our complementary in-vivo, ex-vivo and in-vitro analyses will 1) provide novel insights into the function of Shp2 (and its mutations) in the developing heart; 2) elucidate the functional mechanisms and signaling pathways involved in development of congenital HCM, and 3) suggest potential novel therapies, not only for NS and LS, but for patients with other types of CHD as well.